On the Agency of Water in Volcanic Eruptions, 



129 



But probably the eruption most remarkable for its magnitude, and 

 at the same time for its quiet, was that of Mauna Loa in 1855. In 

 speaking of this eruption Dana says that there was no earthquake, no 

 internal thundering s, no 'premonitions at the base of the mountain. A 

 small glowing point was seen at a height of 12,000 feet, which 

 gradually expanded, throwing off coruscations of light. A vent or 

 fissure then formed, from which a vast body of liquid lava rapidly but 

 quietly flowed during several weeks (a later account says 10 months), 

 forming a stream of lava which extended a distance of 65 miles, with 

 a breadth of from 3 to 10 miles. He adds that those eruptions of 

 fiery cinders which mark so strikingly Vesuvius, are almost wanting 

 about the craters and eruptions of Mauna Loa, and the few that there 

 are, are mainly in connexion with the lateral cones. 



On the other hand, Mr. Scrope remarks that the great paroxysmal 

 eruptions of volcanoes are preceded by earthquakes more or less 

 violent, frequent, and prolonged, "and begin generally with one 

 tremendous burst, which appears to shake the mountain from its 

 foundations. Explosions of aeriform fluids, each producing a low 

 detonation and gradually increasing in violence, succeed one another 

 with great rapidity from the orifice of eruption, which is in most 

 instances the central vent or crater of the mountain." As a con- 

 sequence of such eruptions, the cone is frequently found truncated, 

 " the upper part having been blown off, and in its place a vast chasm 

 formed, of a caldron-like appearance, and of a size proportioned to 

 the violence of the eruption and its duration.* 



One of the most violent of the explosive eruptions was that of 

 the Cosequina in 1835. t This volcano is situated on a promontory 

 south of the Bay of Formosa in Central America. The detonations 

 were so violent that they were heard at a distance of 280 miles. So 

 enormous was the quantity of ashes and scorise shot out of the crater, 

 that for a distance of 25 miles they covered the ground to a depth of 

 about 15 feet, and the finer dust was carried by the wind as far as 

 Jamaica, a distance of 800 miles. It is not recorded that this great 

 outbreak was accompanied by any lava-flow. % The mountain itself is 

 only 480 feet above the sea-level. 



From time to time the violence of other paroxysmal eruptions has 

 blown off and truncated the cone of the volcanoes, and enlarged the 

 craters, from the small dimensions they have when the eruption issues 

 at the finished apex, to gulfs sometimes several miles in circumference 

 and of great depth, eviscerating, as it were, the very centre of the 



* " Volcanos," 2nd Edit., pp. 20-21. 



f The great eruption of Krakatoa has taken place since this was written. It was 

 one of the same character ; we wait the report now preparing by a Committee of the 

 Eoyal Society. 



X Eeclus, '" La Terre," p. 668. 



VOL. XLl. K 



